The technical field of this invention is metallurgy and, in particular, the preparation of metal matrix composites reinforced with ceramic particulates.
Composites formed by the introduction of ceramics into matrices of softer base metals have gained wide acceptance for their cutting and wear resistant properties. The ceramics which are useful for such purposes are typically carbides of refractory metals, such as titanium carbide, tungsten carbide, zirconium carbide and the like. However, the techniques which are presently available for forming such composites are most often cumbersome and ill-suited for the manufacture of articles having complex shapes.
In one conventional process, composites can be formed by powder metallurgy techniques. A fine grained powder of a base metal, such as iron, is typically mixed with a ceramic powder, such as tungsten carbide, and then pressed into a compact. The compact is then sintered at a high temperature to allow interdiffusion between metal-metal and metal-ceramic particles and thereby form a composite in which the ceramic is dispersed through a base metal matrix. Composites have also been formed by sintering three-part powder mixtures of base metal, refractory metal and carbon (i.e. Fe-Ti-C mixtures) in which the carbide is formed by reaction of the refractory metal and carbon at the elevated temperature during sintering.
A number of problems limit the use of powder metallurgy techniques in the formation of hard and wear resistant composites. Considerable effort must be spent in thorough mixing of the powders to assure adequate uniform dispersion of the carbide. The sintering process itself must be monitored carefully to avoid thermal and mechanical stresses which can otherwise result in structural weakness. Moreover, mold design is typically limited to relatively simple shaped articles in order to prevent density differences in the compact and subsequent non-uniform shrinkage during sintering. Finally, sintered parts must often be trimmed or machined into final shape and the nature of refractory ceramic composites can make this particularly difficult; the hardness of the composites can quickly blunt or chip most other cutting tool edges.
Composites have also been formed by mixing ceramic powders directly into a molten or semi-solid base metal in a process known as compocasting. Although reasonably good results have been reported when ceramics have been mixed directly into low melting temperature metals such as aluminum, magnesium or zinc, considerable problems are encountered when the compocasting tecnhique is applied to high melting temperature base metals such as iron. In such instances direct stirring of the ceramic powder is difficult because of density differences and because of the lack of wettability of most ceramics. Direct stirring of ceramic powders into semi-solid slurries is also difficult due to erosion of mechanical stirring devices and non-uniform dispersion of particulates.
Moreover, composites formed by either sintering or compocasting suffer from an additional problem in that they are not well suited for remelting and casting. Typically, the fine dispersion and microstructure of the initial composite is lost during melting. When such composites are melted, the carbide components (i.e., the refractory metal and carbon) go into solution and undergo chemical reactions dependent on composition which most often lead to a modification of the original structure.
There exists a need for better materials for applications in which wear resistance or hardness is important. In particular, techniques which can efficiently produce metal matrix composites with a fine and uniform microstructure, as well as composite materials which can be remelted and cast while retaining their microstructure and structural properties would satisfy a long felt need in the field.